The ventricular system is a network of cavities in the human brain. These cavities are connected and filled with a fluid called the cerebrospinal fluid (CSF); this is the same fluid found in the cavities of the spine. There are four cavities that make up the ventricular system: the left and right ventricles, and the third and fourth ventricles.

If looking at the brain from the front, as when facing a person, there is one large cavity in the right hemisphere of the brain and a matching cavity in the left. These lateral ventricles are separated by a thin membrane called the septum pellucidum and have a distinctive sickle shape. They are identified as the left lateral ventricle and the right lateral ventricle, and each of these structures is also divided into sections. The forefront is known as the frontal horn, which is connected to the occipital horn by a central tunnel. The bottom of the structure that loops back toward the front of the brain is called the temporal horn.

In the center of the partial circle that is made by the lateral ventricles is the third ventricle. This oval-shaped structure is then attached to the fourth ventricle, which sits at the back of the brain above the nape of the neck. The CSF circulates through the chambers of the ventricular system and through the spinal cord, and is constantly being replaced and recycled. The presence of this fluid throughout the ventricular system acts as a natural cushion, in much the same way fluid cushions a developing baby.

The entire ventricular system is lined with a specialized type of cells called the ependymal cells. These cells are responsible for the constant production of CSF, and can also be found throughout the channels in the spine. Collectively, this layer of cells is known as the ependyma. Rarely, there is a type of cancer that develops in these cells called ependymoma, and is usually seen in children.

In the brain of an embryo, there are a number of similar open, empty spaces that develop into the ventricular system as the child grows. The large lateral ventricles and the central third ventricle all develop out of the same basic structure in the embryonic brain. The fourth ventricle develops separately. Another structure at the base of the ventricles, called the central canal, is the section that connects the brain to the spinal cord. This is originally a part of the embryo's spine, and develops into a key channel between the two main bodies.

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